[PPC64] Emit plt call stubs to the text section rather then the plt section.

On PowerPC calls to functions through the plt must be done through a call stub
that is responsible for:
1) Saving the toc pointer to the stack.
2) Loading the target functions address from the plt into both r12 and the
   count register.
3) Indirectly branching to the target function.

Previously we have been emitting these call stubs to the .plt section, however
the .plt section should be reserved for the lazy symbol resolution stubs. This
patch moves the call stubs to the text section by moving the implementation from
writePlt to the thunk framework.

Differential Revision: https://reviews.llvm.org/D46204

llvm-svn: 331607